Distortion correcting device



Jan. 14, 1941. H wEssELs 2,228,862

DISTORTION CORRECTING DEVICE Filed June 2, 1937 L040 INPUT 1 AMP.

INVENTOR HERMA NN WESSELS BY v? ATTORNEY Patented Jan. 14, 1941 PATENTOFFICE DISTORTION CORRECTING DEVICE Hermann Wessels,Berlin-Oberschoneweide, Germany, assignor to Allgemeine Elektricitats-Gesellschaft Application June 2, 1937, Serial No. 146,011 In GermanyJune 6, 1936 5 Claims.

In the art of distant radio communication in which amplifiers are usedthe latter should have in general a rectangular voltage level, that isto say, the amplification should be constant over a certain frequencyrange and drop steeply at the limiting range. In order to attain thiscourse of the amplification the existing curve of the amplifier systemshould be made to conform as nearly as possible to the curve as it oughtto be suitable distortion correcting devices.

The present invention relates to a distortion correcting device which isprovided in the grid circuit of an amplifying tube and which renders itpossible to obtain a nearly constant degree of amplification in therange of the frequency concerned, the amplification decreasing quicklywhen a certain frequency has been attained.

The invention is illustrated by way of example in the accompanyingdrawing in which Figure 1 is a circuit of an amplifier utilizing anarrangement for reducing distortion in accordance with the invention;Figure 2 is an axial section through a particular form of inputtransformer utilized in Figure l; and Figure 3 shows a series ofamplification curves which will serve to explain the invention.

Referring now to Fig. 1 Us denotes an input transformer, the secondaryside of which is composed of two coils 2 and 3. I denotes the primaryside or coil of said transformer. Connected up in parallel with theportion 3 of the secondary coils 2-3 is a condenser C. V denotes theamplification tube. A resistance W is connected between the grid of thistube and the transformer secondary 23.

It is essential for the intended action of the distortion correctingdevice that the coil 3 have a large stray flux with respect to the coil2, as Well as to the coil I. I attain the necessary large stray flux byarranging the coils I and 2 one over the other as shown in Fig. 2, inwhich the coil 2 is wound upon the coil I. This arrangement provides afixed coupling between the coils I and 2 and since the winding 3 islocated laterally with respect to said other two windings l and 2, thenecessary high stray fiux is attained.

Fig. 3 represents a group of curves which show under varying conditionsthe dependency of the degree of the amplification upon the frequency ,f.The curve a shows the course of the degree of the amplification of thecircuit connection of the kind illustrated in Fig. 1, but with omissionof the resistance W. It will be seen that at a certain definitefrequency, resonance due to the stray induction of the coil 3 and thecondenser C takes place. It is desirable for the purpose in view tochoose the resonance frequency in proximity to the limit of thefrequency range which is to be amplified. At this resonance frequencythe am plification attains its maximum value. Besides, a minimum takesplace at another resonance place so that the device can also be used forthe suppression of a frequency band. The curve b shows the variation ofthe amplification with frequency when only the resistance .W isprovided, that is, 10 with omission of the condenser C and thetransformer Us. However, when the circuit connections of the kind shownin Fig. 1 are used, then the curve 0 is obtained, provided theindividual members constituting the device are suitably di- 15mensioned. This curve shows that the degree of the amplification overthe frequency range intended to be transmitted is nearly constant, butdrops sharply at the limit of the transmission range. This drop of thecurve is brought about by the subdivision of the secondary coil in themanner described above.

The results obtained with the invention may be explained further in thefollowing manner. The windings I-2 provide amplification at the lowerfrequencies. As the frequency increases to a point where theamplification would otherwise begin to fall off the tuned winding 3begins to help out and indeed at its resonant frequency would cause thehigh peak of curve a. This peak is due to the coil 3 resonating with Cand also with K, the inherent input interelectrode capacty. Hence byinserting the resistance W the resonant action is damped and by properchoice of W curve 0 can be obtained.

It would also be possible to put resistance W in series with thecondenser C but the position shown is better because in addition to thedamping action the combination of W and K acts as a sort ofpotentiometer that gives the grid a lessening proportion of the totalapplied voltage as the frequency increases. This accounts for thedrooping of the curve b in Fig. 3 at the high frequency end of therange.

The tight coupling between windings I and 2 and the primary resistanceprevents resonant effects from coil 2, while coil 3 is so looselycoupled that the primary resistance has relativeiy little damping effecton the coil 3. Above the natural frequency of coil 3 the voltagereverses in phase which accounts for the big drop in gain as the twotransformer voltages then buck each other.

If the number of turns of the coil 3 is chosen smaller than that of thecoil 2, a steeper drop of the curve a is obtained. In like manner, thecurve can be influenced within certain limits by varying the resistanceW.

I claim:

1. A circuit for obtaining substantially uniform amplification over aWide frequency range and sharp cut-01f at the higher limiting frequencyof said range, comprising a vacuum tube amplifier, a transformer havinga continuous secondary winding connected to the input of the amplifier,a condenser shunting a portion of said secondary Winding, the shuntedportion of said secondary being disposed relatively to the transformerprimary and to the unshunted secondary portion to provide sufficientstray reaotance which forms with the shunt condenser a circuit resonantto a frequency near the limiting frequency of the range to be amplified.

2. A circuit according to the invention defined in claim 1 wherein aresistor is included in the input to the amplifier in series with thesecondary Winding, the efiect of said resistor being such that constantamplification without resonant peaks in the amplification-frequencycharacteristic of the circuit is obtained.

3. A circuit for the amplification of signals without distortioncomprising an electron discharge device, an input transformer thesecondary of which is connected to the input of the discharge device, acondenser shunting a portion of said transformer, the unshunted portionof the transformer being disposed concentrically around the transformerprimary, and the shunted secondary portion being disposed adjacent toand coaxially with respect to said transformer primary.

4. Coupling means comprising a transformer having primary and secondarywindings, the primary winding and a portion of the secondary vnndingbeing disposed to provide close coupling therebetween, the primarywinding and the remaining portion of the secondary winding beingdisposed to provide loose coupling therebetween, and a condenserconnected in shunt to said last mentioned portion of the secondarywinding which is loosely coupled to the primary winding, said condenseradapted to resonate with the leakage reactance of the shunted portion ata predetermined frequency.

5. Coupling means comprising a transformer having a primary winding andtwo serially-connected secondary windings, the primary and one of thesecondary windings being wound concentrically one with respect to theother to provide close coupling therebetween, the other secondarywinding being Wound coaxially with respect to the primary to provideloose coupling therebetween, and a condenser connected in shunt to saidother secondary winding. and adapted to resonate with the leakagereactance of the shunted winding at a predetermined frequency.

HERMANN \VESSELS.

